It's now clear that Hillary Clinton's 'firewall' strategy is alive and well

Hillary Clinton. REUTERS/Jonathan Ernst While Sen. Bernie Sanders' support rose in the polls for months in the early states, many Democrats crowed about Hillary Clinton's "firewall."

The theory went like this: Sanders would not be able to grow his support beyond overwhelmingly white, liberal enclaves among Democratic Party electorates in Iowa and New Hampshire.

"The good thing for her, and the bad thing for Sanders, is that after [Iowa and New Hampshire], I don't see where it gets better for him," a top Democratic strategist unaffiliated with either campaign told Business Insider in January, noting Clinton's black support in South Carolina.

"I just don't see a path for Sanders to win past those two primaries," the strategist said of the Vermont senator.

Now that the Nevada caucuses and South Carolina primary have delivered Clinton huge victories against her rival, those predictions appear to be right.

Hillary Clinton's "firewall" is holding strong.

Exit polls in Nevada and South Carolina showed Clinton with solid leads among female voters, while Sanders again carried voters under the age of 30.

But like Nevada, Clinton's landslide win in South Carolina hinged on black voters, a large part of the Democratic electorate who broke for the former secretary of state by overwhelming margins.

In South Carolina, Clinton captured almost 90% of black voters, a stunning number that surpassed even then-Sen. Barack Obama's support with black voters over Clinton in 2008. The numbers were even higher than the results in Nevada, where Clinton won 75% of the black vote. And they were more significant, as black voters made up more than 60% of the electorate.

The Sanders campaign invested heavily in the first four states, with the hopes that an upset win in Iowa and Nevada would help compel voters to take a serious look at Sanders as an alternative to Clinton.

But even Sanders supporters noted Clinton's narrow wins in those two states may have stymied any "momentum" the campaign wanted to gain heading into Super Tuesday, when more than 10 states will vote. Senior Sanders strategist Tad Devine admitted recently that the narrow losses in Iowa and Nevada loss may made Sanders' path to victory more difficult.

As returns suggested a landslide victory for Clinton in South Carolina, some observers said the win forecast similar results on Super Tuesday, as the Democratic electorate is much more racially diverse in the participating states.

"SC results are very clear, absent Sanders dramatically expanding his base on Super Tuesday, the Dem nomination fight is over on Wednesday AM," Dan Pfeiffer, a former senior adviser to Obama, tweeted shortly after Clinton's win.